Who said you need a muffler!Just get you pipes shooting straight out the top.

Like was stated above ,super trapps are very compact.They mount at the end of your tubes.They are also tunable,you can get as loud or as quite as possible.If it were me I would leave that until the end of the project then you can get under the car and see what you have room for.Mufflers come in all shapes and sizes.The only one that will no for sure what you need will be you.
Do you want something quite ,or something loud?